If the pattern saves you time, (vs figuring out what you want, trying it, adjusting it etc), fabric (because the pieces are cut the right size the first time) and energy, then it's worth it.
Also, there may be features you hadn't thought of, or techniques you haven't seen/used that make it a well constructed bag.
And I particularly like the idea of sharing the pattern with someone.
If I like a particular bag pattern, I will make it often.